I need to turn one side of a cylinder into a square a leave the other side round
Don't know if this is what you wanted, but you may just bridge the vertices between square and circle to create this kind of shape. Add a plane and subdivide it as many times as you want. Then add a circle with the number of vertices matching the number of the vertices on the edges of the square/plane. Finally select them as pictured below, press Spacebar and type Bridge-->Bridge Edge Loops.

NOTE: There is similar answer about this here on BSE, but I can't find it.